The present invention pertains to the manufacture of display panels and especially plasma panels.
A plasma panel has two dielectric plates each comprising an array of conductive electrodes. These plates, formed out of a glass substrate, are joined to each other in an imperviously sealed manner, the arrays of electrodes being substantially orthogonal. The two plates demarcate a gas-filled space. Each intersection of electrodes defines a cell where discharges can be made into the gas.
In this type of display panel, the plates comprise a dielectric layer that is typically an enamel type of layer insulating the electrodes from the gas. This dielectric layer limits the discharge current by storing the charges created by ionization and gives the panel a memory effect.
By supplying the electrodes with an alternating sustaining signal, it is possible to dissociate the functions of addressing and light energy generation thus making it possible to build large-sized panels. This type of display panel has high technological simplicity and is particularly robust.
The dielectric layer is generally made by means of an enamel based on lead oxide, silica and boron oxide (PbO, SiO2, B2O3). The dielectric layer placed on the front plate of the panel, namely the face seen by an observer, is generally transparent while the layer placed on the rear plate is generally white in order to return a maximum amount of light towards the front.
It is standard practice to cover the dielectric surface of the plates with a deposit based on magnesium oxide (MgO) to protect it from bombardment by the ions of the gas, reduce the voltage needed for a discharge to be produced and ensure the temporal stability of the voltage needed for the discharge.
In color plasma panels, the gas is generally a mixture of neon, xenon and helium. The ionizing of the gas prompts an emission of ultraviolet rays that excites a mosaic of luminophors deposited on the magnesium oxide based layer of one of the two plates between two electrodes of a network.
The magnesium oxide based deposit must have a high secondary transmission coefficient and must provide perfect electrical insulation. The making of this deposit should not cause any deterioration in the dielectric plate. The magnesium oxide based deposit must withstand the subsequent treatment to which the plate has to be subjected without undergoing dissociation while at the same time playing its role of protecting the dielectric layer. The subsequent operations of treatment to which a plate is subjected are: the depositing of the luminophors that integrate aqueous phase treatment operations, and the sealing operation which is done at temperatures of about 400xc2x0 C.
At present, the magnesium oxide based deposition is done by vacuum evaporation. This operation requires a secondary vacuum chamber working with a pressure lower than or equal to 10xe2x88x925 hectopascals. An electron gun is placed in the vacuum chamber. It heats the magnesium oxide by bombarding it, thus prompting its evaporation and its deposition on the dielectric plate. The deposition is done at about 100xc2x0 C. The installation used for this deposition is costly. The duration of the deposition is relatively lengthy, for thicknesses of about 300 to 500 nanometers are required and the production rates cannot be high.
This operation of deposition which is already costly for medium-sized plates (with a 49 centimeter diagonal) becomes even greater for large-sized plates whereas one of the promising features of the plasma display panels is that it is quite adapted to large formats (with a diagonal of 150 centimeters or even more).
Another major drawback of this method of deposition by vacuum evaporation relates to the quality of the deposition. The inventors have observed that the deposits obtained are mechanically and chemically not stable enough, deteriorate under the effect of ion bombardment, especially under the effect of xenon ion bombardment and do not properly play their role of protection during the deposition of the luminophors. Indeed, it turns out to be the case that deposition by vacuum evaporation leads to a low density magnesium oxide based layer with relatively large intergranular spaces making it permeable to water. This explains firstly its sensitivity to ion bombardment and secondly its insufficiency in properly protecting the dielectric layer during the aqueous phase treatment. The water, by infiltrating into the pores thus formed, is able to reach the dielectric layer and cause damage to it. Measurements have shown porosity levels of about 30% to 40%.
Ion effects and the deterioration of the dielectric layer leads to pollution of magnesium oxide, the contamination of the interior of the gas space and an insufficient lifetime for the panels thus made.
It is an object of the present invention to propose a method for carrying out a magnesium oxide based deposition on the dielectric surface of a glass plate of a display panel that leads to a layer that is waterproof in such a way that it is practically insensitive to ion effects and does not cause any deterioration of the plate during the deposition or during subsequent operations of treatment to which the plate may be subjected. In particular, it must be borne in mind that the dielectric surface of the plate cannot be heated intensely, beyond about 430xc2x0 C. Otherwise, there would be deformation, and this is unacceptable.
To achieve this result, the method according to the invention comprises at least the following steps:
the creation of a mist from a metalorganic compound of magnesium dissolved in a solvent,
the conveying of the mist to the dielectric surface of the plate,
the evaporation of the solvent when approaching the dielectric surface of the plate which is taken to a temperature of about 380xc2x0 C. to 430xc2x0 C.,
the pyrolysis of the metalorganic compound leading to the magnesium oxide based deposit on the surface of the plate and the evaporation of the organic radical of the compound, this deposit being practically waterproof.
As a metalorganic compound, it is possible to use magnesium acetate or magnesium acetonate acetyl.
An organic solvent such as butanol or methanol for example may be chosen.
The mist is obtained preferably by means of an ultrasound generator plunged into the solution. A particularly homogeneous nebulization is then obtained.
The mist is conveyed towards the dielectric surface of the plate by means of a vector gas such as air, pure oxygen or a mixture of nitrogen and oxygen. This gas contributes to the transformation of magnesium into magnesium oxide by oxidation.
It is preferable that the mist should be preheated when it is being conveyed, this preheating taking place preferably in a conduit through which the mist passes. In an installation of continuous disposition, it would be useful for the plate to be capable of shifting with respect to a nozzle by which the conduit ends.
A case may be envisaged where the magnesium oxide based deposition contains a dopant to improve its electrical properties. The dopant may be, for example, calcium oxide, yttrium oxide or barium oxide.
In this variant, the solution will contain a metalorganic compound whose metal radical corresponds to the dopant.